Understanding Map Projections

PROJECTED COORDINATE SYSTEMS
A projected coordinate system is defined on a flat,
two-dimensional surface. Unlike a geographic
coordinate system, a projected coordinate system has
constant lengths, angles, and areas across the two
dimensions. A projected coordinate system is always
based on a geographic coordinate system that is
based on a sphere or spheroid.
In a projected coordinate system, locations are
identified by x,y coordinates on a grid, with the
origin at the center of the grid. Each position has
two values that reference it to that central location.
One specifies its horizontal position and the other its
vertical position. The two values are called the
x-coordinate and y-coordinate. Using this notation,
the coordinates at the origin are x = 0 and y = 0.
On a gridded network of equally spaced horizontal
and vertical lines, the horizontal line in the center is
called the x-axis and the central vertical line is called
the y-axis. Units are consistent and equally spaced
across the full range of x and y. Horizontal lines
above the origin and vertical lines to the right of the
origin have positive values; those below or to the left
have negative values. The four quadrants represent
the four possible combinations of positive and
negative x- and y-coordinates.
The signs of x,y coordinates in a projected coordinate system.
10 • Understanding Map Projections